The following lines are already in the wp-config.php file, and you should always change the default values.
A way to do this is going to https://api.wordpress.org/secret-key/1.1/salt/ and replace the default values with the ones given.

You can set Automatic Database Optimizing. If a database is corrupt normally the user isn’t able to login so to run this script the user doesn’t need to be logged in. It’s recommended to delete this line after you repair your database.
After including the line in your wp-config.php the script can be found at http://your_domain/wp-admin/maint/repair.php